1. Field of the Invention
The present invention relates to optical signals and to optical signal generation for use in optical communications, and finds particular application in optical time division multiplexing (OTDM).
2. Related Art
The use-of OTDM signals currently offers both access to aggregate data capacities beyond the reach of commercial broadband electronics, and the additional flexibility of optical routing without recourse to high capacity electronic switches.
Typically, short pulses may be encoded and interleaved to produce a traditional OTDM data sequence, or modulators may be used to shape pulses and form an optically multiplexed signal by combining several such sequences. Both of these optical techniques require multiple optical paths and accurate synchronisation of the optical path lengths. It is also important in an OTDM interleaver in these known arrangements to exhibit a high enough extinction ratio to avoid interference effects at its output between data channels. Furthermore, the maximum line rate (eg 100 GHz) of an OTDM system is determined to a large extent by the width of the base rate leg 10 GHz) pulses, as highlighted in xe2x80x9cTransmission of a true single polarisation 40 Gbitis soliton data signal.xe2x80x9d, Electronics Letters, vol. 29, no. 11, pp990-992.
One alternative method of producing an OTDM signal is described in xe2x80x9cAll-optical time division multiplexing using four-wave mixingxe2x80x9d, Electronics Letters, vol. 30, no. 20, pp 1697-1698. In this paper OTDM is achieved by modifying a 100 GHz 1547 nm optical signal by time-delayed 6.3 Gbit/s signals to generate sub-channels in a 1557 nm 100 Gbit/s signal via four-wave mixing. This method requires a series of wavelength division multiplexers each adding a sub-channel, or data channel, to an OTDM stream. Whilst this method relaxes the constraints on the extinction ratio of the 100 GHz signal, precise control of optical delays is still required.
This and other methods known to the applicants deal only with what may be termed xe2x80x9cbright pulsexe2x80x9d OTDM transmission.
The present inventors have realised that it is both possible and advantageous to implement optical communications systems in which xe2x80x9cdark pulsesxe2x80x9d replace xe2x80x9cbright pulsesxe2x80x9d as the information-bearing component.
According to one aspect, the present invention provides a method of generating an optical data signal, wherein dark pulses representative of one or more data sequences are imposed by at least two dark pulse generators onto an optical input signal received by said generators for subsequent transmission along an optical fibre, the dark pulse generators being in optically coupled alignment with the optical input.
The optical input may comprise a substantially continuous burst of optical radiation, such as might be provided by the output of a continuous wave optical signal generator. Alternatively, the optical input may comprise a pulse train such as might be provided by an optical clock. The effect on an incoming pulse train such as that provided by an optical clock might then be that bright pulses are missing from the pulse train.
In this specification, a xe2x80x9cdark pulsexe2x80x9d is a temporal gap, or region of reduced intensity radiation, in incoming optical radiation, or light beam. (Although generally in optical communications, the emphasis lies on speed and therefore short pulse lengths are advantageous, the term xe2x80x9cdark pulsexe2x80x9d should not be taken to indicate of itself a limitation on the length of the temporal gap, or region of reduced intensity radiation,)
An advantage of using dark pulses in place of bright pulses is that optical signal generation may be simplified, as will be discussed in the subsequent description.
Each of the dark pulse generators may provide a respective data signal and these may be interleaved so as together to provide an OTDM signal. A particularly convenient way of providing the interleaving is to fabricate the dark pulse generators on a common substrate, optically aligned so that the output of one, carrying its dark pulse data train, is fed straight to the next which can then add its own in a different time slot of the OTDM signal. As is further discussed below, this avoids the use of optical delay lines although it is still necessary to provide electrical synchronisation between the dark pulse generators.
(It should be noted that embodiments of the invention are not limited to OTDM however as there may clearly be other applications which benefit from the present invention. It would be possible for instance to produce a single output signal by means of consecutive dark pulses being generated by consecutive dark pulse generators. Such an arrangement may still benefit from a speed advantage.) Use of dark pulses in OTDM is particularly advantageous. While pulse alignment remains important for dark pulse OTDM, the extinction ratio is less of a problem than in bright pulse OTDM. This is because, in bright pulse OTDM, there have to be provided multiple parallel optical paths so that each pulse generator can add its pulses without xe2x80x9cblotting outxe2x80x9d pulses imposed by another pulse generator. These multiple optical paths have to be recombined and that causes interference problems because of phase variations in the background. The random interference causes errors unless there is an extinction ratio of about for instance 40 dB in a four channel system. In dark pulse OTDM however, there is only the one optical path, therefore there does not have to be recombination of the paths and the interference effects described simply do not happen. Although there is still a constraint on the extinction ratio in dark pulse OTDM, it is a question of the power budget at the receiver. A reasonable extinction ratio in a four channel system for dark pulse OTDM is more likely to be of the order of 15 dB.
In a preferred embodiment, each one of the dark pulse generators generates dark pulses for just one data channel of the OTDM signal. Electronics would thus only limit the data rate of a single data channel. The overall optical signal data rate could then be well beyond the aggregate data rate of commercial broadband electronics.
Preferably, a dark pulse generator comprises an optical modulator having both high and low optical transmission states and a high optical extinction ratio (although, as discussed above, not as high as that required for bright pulse OTDM). The operation in one or other state may for example be determined by an electrical bias signal. The optical extinction ratio, as discussed above, could for instance be signicantly less than 40 dB in a four data channel system, for instance lying in the range 10 to 30 dB, and a reasonable value being of the order of 15 dB.
The applicants have shown that a suitable optical modulator is an electro-absorption modulator (EAM). A suitable electrical bias signal for an EAM comprises clock (for example a sinewave) and data components combined, for example, using a simple power splitting arrangement, where a dark pulse is formed when both clock and data components are negative. This arrangement obviates the need for signal processing in the electrical domain which would otherwise be necessary to provide a suitable data-encoded bias signal.
Preferably, an EAM is biased to provide high optical extinction for a short period of time to encode a dark pulse onto a light beam input. When no data is present, the electrical signal is arranged to bias the EAM to be in its high optical transmission state.
In a preferred embodiment of the present invention, a plurality of EAMs are optically cascaded and are arranged to generate dark pulses on a light beam from a single light source.
Preferably, each EAM is arranged to generate one OTDM channel. This arrangement has the advantage that each EAM leaves light substantially unperturbed between the dark pulses which it generates, and each subsequent modulator in the cascade can then modulate the unperturbed light. Thus, with suitable electrical timing it is possible to generate a high-speed OTDM data signal.
In theory, any number of EAMs may be cascaded to provide any number of data channels of an OTDM signal from a single light source. In contrast, other known methods, using multiple EAMs and bright pulses, would require either multiple light sources or one light source with multiple outputs to provide a comparable OTDM signal.
In practice, the number of EAMs which could be cascaded in accordance with the present invention would be limited by the shortest pulse width available from each modulator. For example, if the shortest dark pulse that can be generated is 10 ps, then only ten modulators can be placed in a row (at 10 Gbit/s) to produce 100 Gbit/s data. However, if dark pulses having widths less than 5 ps can be generated, then twice as many modulators may be cascaded.
A further limitation on the number of modulators which may be cascaded is due to optical loss incurred by each EAM, since an EAM does in fact incur some loss even when in its high optical transmission state. However, such loss could be compensated for by including optical amplification between one or more of the EAMs. For example, one or more optical fibre amplifiers, such as rare-earth doped fibre amplifiers using erbium or praseodymium could be used to provide amplification between EAMs.
Advantageously, the use of optically cascaded EAMs allows time slot alignment to be carried out substantially only in the electrical domain, when using the EAMs to generate dark pulses. Thus, optical delay lines can generally be dispensed with to provide time slot alignment.
In a particularly advantageous form of the invention, a plurality of EAMs are integrated by forming the EAMs, in cascaded optical alignment, on a single semiconductor (for example InP) substrate. Preferably also, a single light source may be integrated onto the same substrate, in optical alignment with the cascaded modulators, to provide a single OTDM signal generating device. Advantageously, the whole optical system for an OTDM signal generator could thus be provided as a single semiconductor device. Such a device could be relatively cheap and compact compared to known devices or systems. In any of these xe2x80x9cintegratedxe2x80x9d arrangements, it would of course be possible to use semiconductor optical amplifiers between adjacent ones of the EAMs.
An alternative dark-pulse generator to an EAM could be an optical AND gate, for example an AND gate implemented using a non-linear optical loop mirror (NOLM), operating with a switching window (provided by an optical switching signal) which switches light transmission from a main transmission path to an alternate path for the period of one pulse width. However, a NOLM typically requires a long length of optical fibre incorporating some non-linearity, such as a non-linear semiconductor device or a doped optical fibre, to provide switching by known phase modulation effects. Such an arrangement would not be so convenient or robust as the EAM arrangement described above. Also, the switching window for a NOLM would need to be provided by an optical signal and could not be generated directly by an electrical signal. Thus, such a system would need two optical stages at least for pulse generation (optical pulse formation stage and optical window stage), mitigating the advantages of using dark pulses generated by, for example, EAMs.
In accordance with a second aspect, the present invention provides an optical modulator for use in OTDM, the modulator comprising at least two optically cascaded dark pulse generators, for generating dark pulses representative of one or more data sequences on an optical input signal coupled from a light source to a first of said generators.
The optical input signal may for instance comprise a substantially continuous optical beam, or an optical clock comprising a series of bright pulses.
In accordance with a third aspect, the present invention provides an optical signal generator comprising:
a light source;
a plurality of means for generating dark pulses representative of one or more data sequences onto an optical input signal provided by the light source, said means being in optically coupled alignment, a first of said means being arranged to receive light from the light source; and
means for coupling light from a last of said means into an optical fibre communication system
Again, the optical input signal may for instance be substantially continuous or may be a clock pulse train, comprising a regular series of bright pulses.
In accordance with a fourth aspect, the present invention provides a semiconductor device comprising:
a semiconductor substrate onto which are fabricated at least two electro-absorption modulators in optically coupled alignment, and
means for providing electrical drive to each modulator,
wherein each electro-absorption modulator and its electrical drive can be arranged, in use, to generate dark pulses representative of one or more data sequences onto a light beam input to a first of said modulators.
It may be convenient if a source for the light beam is also fabricated on the semiconductor substrate, in optically coupled alignment with the modulators.
In accordance with a fifth aspect, the present invention provides an optical signal comprising a dark pulse data train, wherein the signal is segmented into time slots to provide an optically time division multiplexed (OTDM) data sequence.
In accordance with a sixth aspect, the present invention provides an optical communications network including at least one signal generating arrangement according to any of the earlier aspects of the invention.
Whilst the use of embodiments of the invention with either a substantially continuous incoming light beam, or with some other input such as a bright pulse input, is discussed above, it will be realised that there may be a requirement for correct timing between a non-continuous input and the dark pulses to be imposed. For instance, there would clearly have to be synchronisation between an incoming clock signal and the dark pulses if the dark pulses are to remove selected bright pulses from the clock signal. This can be provided for instance by the electrical bias signal(s) in the case of EAMs.
In accordance with a further aspect of the present invention, there is provided an optical modulator comprising:
an optical clock source;
a plurality of means for selectively switching off light received from the clock source in order to generate dark pulses representative of one or more data sequences, said means being in optically coupled alignment, and a first of said means being arranged to receive light from the clock source; and
means for coupling light from a last of said means into an optical fibre communication system.
In accordance with a still further aspect of the present invention, there is provided an optical modulator comprising:
i) an electro-absorption modulator having high and low optical transmission states;
ii) an electrical bias input to the electro-absorption modulator; and
iii) an electrical data input to the electro-absorption modulator;
wherein the data input combines with the bias input to change the state of the electro-absorption modulator at least temporarily to a low transmission state, the change in transmission state generating a dark pulse representative of one or more data sequences.